1. Field of the Invention
The present invention relates to buck converters, switching regulators and power converters.
2. Description of Related Art
In applications for computers, communication and consumer products, buck converters are widely applied to provide regulated output voltages such as 3.3V, 2.5V, 1.8V, 0.9V etc. from a higher input voltage, such as 12V. Among these applications, PWM (pulse width modulation) is a switching technology mainly used. In addition, a synchronous rectifying technology is applied to improve the efficiency of buck converters under heavy load conditions.
FIG. 1 shows a conventional buck converter. A switch 20 is coupled to an input voltage VIN of the buck converter. With the operation of the switch 20, the power of the input voltage VIN will be converted to an output voltage VO via an inductor 30. A voltage divider formed by resistors 51 and 52 is coupled from the output voltage VO of the buck converter to a ground reference level. The voltage divider provides a signal VFB, which is proportional to the output voltage VO, to a control circuit 10. The control circuit 10 outputs a switching signal SW1 in response to the signal VFB to turn on/off the switch 20 to achieve the voltage regulation. A diode 22 is coupled between the switch 20 and the ground reference level to cycle a current of the inductor 30 when the switch 20 is turned off.
FIG. 2 is a known buck converter having synchronous rectifying function. A switch 25, which acts as a synchronous rectifier, is used to reduce the power loss caused by a voltage drop of the diode 22. The diode 22 in FIG. 1 can be substituted by a parasitic diode 23 of the switch 25 or by an added Schottky diode. A switching signal SW2, which is used for turning on/off the switch 25, is in inverse logic to the switching signal SW1.
FIG. 3A˜3D show the synchronous rectifying operation of the buck converter. Referring to FIG. 3A, when the switch 20 is turned on, a current IC is sourced from an input voltage VIN to an output capacitor 40 via the inductor 30 for generating the output voltage VO. Therefore, the output voltage VO of the buck converter is obtained across the output capacitor 40, while energy is stored into the inductor 30 and the capacitor 40.
As shown in FIG. 3B, when the switch 20 and the switch 25 are both turned off, energy stored in the inductor 30 will be continuously supplied to the output of the buck converter via the parasitic diode 23 of the switch 25. After that, the switch 25 is turned on to reduce the power loss of its parasitic diode 23 as shown in FIG. 3C. Before turning on the switch 20 again for the next switching cycle, the switch 25 is turned off in advance to prevent short circuit due to cross conduction. Notwithstanding, the synchronous rectifying operation can improve the efficiency of the buck converter at heavy load condition but fails to overcome low efficiency at light load condition, which is caused by a backward discharging phenomenon. As FIG. 4 shows, under light load conditions, the energy stored in the inductor 30 will be fully discharged before the next switching cycle starts. The energy stored in the capacitor 40 will therefore be backward discharged via the inductor 30 and the switch 25.
Furthermore, under light load conditions, the major power losses of the buck converter are in direct proportion to the switching frequency F of the switching signal SW1, such as the core loss of the inductor 30 and the switching losses of the switches 20 and 25. Another major loss of the buck converter is caused by the power consumption of the control circuit 10. A switching period T is a reciprocal of the switching frequency F, which can be shown as following equation:T=1/F=(TON+TOFF)Where TON and TOFF are respectively an on-time and an off-time of the switching signal SW1.
Increasing the switching period T reduces the power losses. However, in order to shrink the size of inductors and capacitors, the switching frequency F is restricted to operate in a short switching period. A maximum on-time is also limited to prevent the saturation of the inductor. Therefore, increasing the off-time TOFF can extend the switching period T for light load conditions. Therefore, the power consumption of the buck converter is reduced in response to the increase of the switching period T under the light load and even no-load conditions.